Radio frequency (RF) phased arrays have been widely used in military radar and wireless communication systems. Advancements in semiconductor processing technology have enabled monolithic realization of RF phased arrays for commercial applications such as automotive radars and high-speed wireless communications.
Optical phased arrays enable imaging, lidar, display, sensing, and holography. For instance, low-cost large-scale optical phased arrays enable high-resolution imaging and ranging capabilities for automobiles, autonomous vehicles, drones, etc. Optical phased arrays enable 3D cameras. Optical phased arrays can be used for projection and display applications. Dynamic holography is also enabled by two-dimensional optical phased arrays. Optical phased arrays enable high-speed communications including those for chip-to-chip, board-to-board, free-space, and terrestrial applications. Optical phased array can eliminate the stringent mechanical requirement for optical alignment and positioning, and enable reconfigurability in the communication link.
Historically, optical beam steering has been achieved mechanically through, for instance, rotating or tilting mirrors that reflect the light towards different directions. Limitations of mechanical beam-steering include (1) low steering speed, (2) low reliability of moving parts, (3) large form factor, (4) high power consumption, and (5) high cost.
Another traditional beam-steering approach is based on changing the reflection or transmission angles of optical components that are based on liquid crystals or similar material where the refractive index changes with application of electrical stimuli.
Advancements in semiconductor nanofabrication enable realization of complex monolithic optical chips that include a large number of compact optical and even electrical components. For instance, complex optical integrated systems are used in commercial fiber optical communications.
Monolithic optical phased arrays with various complexities using different semiconductor technologies and platforms have been reported. In most past demonstrations, grating couplers are used as optical radiators (aka optical antennas). Beam-steering is done either using variable phase shifters or wavelength scanning.
The major limitations of state-of-the-art optical phased arrays are large area and power consumption of unit optical elements. A unit optical element of a phased array includes an optical variable phase shifter and an optical radiator (antenna). The large footprint of unit elements results in designs with large spacing between elements; this introduces grating lobes and limits the unambiguous field of view. Large power consumption of unit elements prohibits realization of large-scale optical phased arrays at a reasonable power consumption. The fundamental reason for both of the aforementioned limitations is the weak thermo-optical and electro-optical effects in semiconductors. In reported arrays, optical variable phase shifters are based on changing the optical propagation velocity using thermo-optical or electro-optical effects. Weak effects necessitate increasing either the electrical or thermal strengths or device length, leading to increasing power consumption or area, respectively. Optical resonators may be used to increase the sensitivity of optical phase shift with the change of refractive index; however, additional control mechanisms, with associated power consumption and area, are required to ensure that the resonator-based phase shifters operate at the desired wavelength across manufacturing process and temperature variations. Non-semiconductor material may be used to realize variable optical phase shifters with possibly less area and/or power consumption. However, these new materials are largely incompatible with commercial semiconductor processes, and cannot be used towards realization of large-scale optical phased arrays with high manufacturing yield.
Another challenge is the optical loss associated with optical waveguides and components. In large-scale optical phased arrays, the signal would travel several millimeters, through optical components such as optical power dividers, optical phase shifters, optical radiators, etc. each with a non-zero optical loss. Optical amplification may be required to compensate for the aforementioned loss. However, currently, compact power-efficient low-noise optical amplifiers and monolithic optical phased array architectures that benefit from them do not exist.
In summary, large-scale two-dimensional optical phased arrays, with small spacing between optical radiators (ideally half wavelength), compatible with commercial semiconductor processing technologies do not exist.